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Transcript of Shipboard determination of hydrogen peroxide in the western Mediterranean sea using flow injection...
Shipboard determination of hydrogen peroxide in thewestern Mediterranean sea using ¯ow injection with
chemiluminescence detection1
David Price2,a,b, R. Fauzi C. Mantourab, Paul J. Worsfolda,*
a Department of Environmental Sciences, Plymouth Environmental Research Centre, University of Plymouth, Plymouth PL4 8AA, UKb Plymouth Marine Laboratory, Centre for Coastal and Marine Sciences, Prospect Place, West Hoe, Plymouth PL1 3DH, UK
Received 10 March 1998; received in revised form 7 May 1998; accepted 11 May 1998
Abstract
Field studies at a variety of sampling locations in the western Mediterranean have endorsed the use of a novel ¯ow injection-
chemiluminescence method (FI-CL) for the determination of hydrogen peroxide in seawater. Analysis time for a depth pro®le
of 12 samples including standard additions was 45 min, repeatability (relative standard deviation; RSD; n�5) was typically
<5% and reproducibilities at two different sampling stations were 2.5% RSD (mean concentration�83.6 nM H2O2; n�3) and
1.8% RSD (mean concentration�42.5 nM H2O2; n�4). Depth pro®le data, characterised by H2O2 concentrated at the surface
(16.0±154 nM H2O2) and decreasing rapidly below the thermocline, were in agreement with previous studies in similar
oceanographic environments. Samples collected during Lagrangian drift mode indicated that diurnal maxima can occur in the
mid-late afternoon. Batches of sampled and treated seawater were incubated under ambient light conditions whilst onboard
ship to investigate the photogeneration of H2O2. Surface waters had a higher H2O2 photogenerative ef®ciency than deeper
waters and samples collected close to the River RhoÃne plume were particularly effective at generating H2O2, suggesting that
the chromophoric material responsible for the photogeneration of H2O2 had a terrestrial source. Additional incubation
experiments in the laboratory showed that humic material was more ef®cient at photogenerating H2O2 than fulvic material or
hydrophilic macromolecular acids. # 1998 Elsevier Science B.V. All rights reserved.
Keywords: Shipboard ¯ow injection; Hydrogen peroxide; Seawater; Western Mediterranean Sea
1. Introduction
Hydrogen peroxide is thought to play a key role in
redox reactions in marine environments [1,2], parti-
cularly in surface waters (top 10 m) where concentra-
tions in the range 100±1000 nM have been reported
[3]. Hydrogen peroxide is therefore likely to govern
the redox speciation of several trace metals and parti-
cipate in oxidation reactions involving labile and
residual organic compounds. It is predominantly
formed by photochemical generation via free radical
intermediates and dissolved organic chromophores
[4,5] and is found in all natural aquatic environments
[6±10]. Wet and dry deposition of photogenerated
Analytica Chimica Acta 377 (1998) 145±155
*Corresponding author. Tel.: +44-1752-233006; fax: +44-1752-
233009; e-mail: [email protected] of original article: S0003-2670 (98) 00322-5. This article
has previously been published in 371/2-3.2Present address: Department of Chemistry and Biochemistry,
Old Dominion University, Norfolk, VA 23529-0126, USA.
0003-2670/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved.
P I I : S 0 0 0 3 - 2 6 7 0 ( 9 8 ) 0 0 6 2 1 - 7
H2O2 in the atmosphere can be a signi®cant transient
source to the oceans [7,11] and biological production
by certain phytoplankton provides a minor input
[12,13]. The role of biota in enzyme-mediated decom-
position of H2O2 [14] however is a more signi®cant
factor than photochemical decomposition [15] in the
breakdown of H2O2 in seawater.
The transient nature of H2O2 in marine waters
necessitates a rapid, ®eld-based analytical method
for its determination. A previous report [16] showed
that ¯ow injection with chemiluminescence (FI-CL)
detection was a robust and reliable technique for the
shipboard determination of H2O2, with a detection
limit (S/N�3) in seawater of 5 nM, a linear range of 5±
500 nM and relative standard deviations (RDSs)
(n�5) typically in the range 1±2%.
This paper presents further validation of the FI-CL
technique for shipboard deployment in marine envir-
onments and discusses ®eld data (particularly H2O2
depth pro®les) from a four-week cruise in the Med-
iterranean Sea. The photogeneration of H2O2 in dif-
ferent marine waters is also investigated using
shipboard and laboratory based incubation.
2. Experimental
2.1. Sampling
Seawater samples from various depths and loca-
tions in the western Mediterranean were collected
using 10 l PTFE-lined Go-Flo bottles (General Ocea-
nics, Fort Lauderdale, FL) and immediately trans-
ferred to clean amber 50 ml glass bottles (soaked in
2 M HNO3; well rinsed with Milli-Q water). Each
sample bottle was rinsed three times with sample
before collection of the true sample and analysis
was performed within 1 h of sample collection.
The relevant Stations for Cruise D203 onboard
R.R.S. Discovery are shown in Fig. 1(a) (leg 1) and
(b) (leg 2). The cruise embarked at Gibraltar on 2nd
July 1993 and terminated on 2nd August 1993 at Nice
in southern France. The track for leg 1 was west
through the Straits of Gibralter to Station D3 then
east via Stations D5-9 to the Straits of Sicily (arriving
at Station D10 on 9th July) and north-west to Station
MA1, arriving in Monaco on 15th July. The track for
leg 2 was from Monaco (on 17th July) back to Station
MA1, where R.R.S. Discovery remained in Lagran-
gian drift mode for almost three days (Casts MA1/1 to
MA1/26), then to Roustan Bouy (Station MA9), arriv-
ing on 21st July. The RhoÃne plume area was studied in
detail (Stations MA2-8, MF1-3 and ME1-2) before
departing for a second Lagrangian drift Station at
MD1 from 30th July to 1st August and then to Nice.
2.2. Reagents and standards
All solutions were prepared from ultra-pure deio-
nised water supplied by Milli-Q systems (Millipore,
Milford, MA) in the laboratory and at sea. Luminol,
cobalt(II) nitrate hexahydrate and 30% H2O2 were
analytical grade and supplied by Fluka (Gillingham,
Dorset, UK). All other chemicals, unless otherwise
stated, were reagent grade and supplied by Merck
(Poole, Dorset, UK).
Stock solutions of luminol (1�10ÿ3 M) and cobalt
(II) nitrate (1�10ÿ2 M) were prepared in 0.1 M
sodium carbonate, adjusted to the required pH with
2 M HCl and stored in high density polyethylene
(HDPE) containers. Working solutions were prepared
by serial dilution and stored at 48C to minimise
decomposition. The luminol solution was left for
24 h before use to ensure maximum CL activity.
Three size fractionated seawater samples for ship-
board incubation experiments were kindly donated by
G. Fengler from the University of Hamburg during
Cruise D203. The samples were obtained by collecting
105 l of surface seawater (1.6 m depth) from Station
MA9/10 (4.98E, 43.38N) at 2158 on 27th July 1993.
The seawater was suction ®ltered to remove particu-
late matter (>0.2 mm) and 94.0 l of the ®ltrate was
passed through two modi®ed tangential ®ltration sys-
tems (DT-Modul, Rochem) ®tted with 8000 and
300 Da membrane sizes. Both membranes were com-
posed of modi®ed polyethersulfone and the water
forced through the ®ltration system by a gear pump
(Tuthill). Operating pressures for the 8000 and 300 Da
membranes were 5 and 10 bar respectively. The mem-
brane-retained material was quantitatively transferred
by rinsing with water. Four fractions were obtained
(®ltered seawater (11 l), >8000 Da (redissolved in
1.98 l water), 300±8000 Da (redissolved in 3.92 l
water) and <300 Da (94 l)).
Natural water organic material for incubations in
the laboratory was obtained from three samples col-
146 D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155
lected from the River Dodder, Eire. Filtration, adsorp-
tion chromatography (using Amberlite XAD-4 and
Amberlite XAD-8) and ion exchange chromatography
were used to fractionate and purify H-saturated forms
of humic, fulvic and hydrophilic macromolecular
acids [17]. Each fraction was freeze-dried to allow
quantitative preparation of solutions of known con-
centration. A commercial ionic humic acid salt
Fig. 1. Sampling stations for Cruise D203 onboard R.R.S. Discovery. (a) leg 1 (b) leg 2.
D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155 147
(Aldrich, Gillingham, Dorset, UK) was obtained and
used as supplied.
2.3. Instrumentation and procedures
Hydrogen peroxide measurements were made using
an automated FI-CL system described in detail else-
where [16]. The optimised FI manifold is shown in
Fig. 2. The cobalt(II) and luminol streams were
pumped at 1.7 ml minÿ1, merged at a perspex T-piece
and passed into a coiled glass ¯ow cell situated
immediately in front of the photomultiplier tube
(PMT) end window. Seawater samples and standards
(100 ml) were injected into the luminol stream prior to
merging. The chemiluminescence emission
(�max�440 nm) resulting from the oxidation of lumi-
nol was directly proportional to the concentration of
H2O2 and was output to a chart recorder. A standard
additions method (three additions) was used to quan-
tify H2O2 and the order of analysis was kept random to
avoid systematic errors.
2.4. Laboratory and deck based incubations
Acid-cleaned sealed quartz glass tubes and sintered
caps (volume�45±50 ml) were used for all incubation
experiments. During the laboratory studies the tubes
were secured in a thermostated (3±58C) sample tray
supplied with an accelerated exposure unit designed to
simulate global radiation (Suntest CPS, W.C. Heraeus
GmbH, Hanau, Germany). At sea the tubes were held
in a purpose-made rigid PVC tube rack (built in house)
and situated in an unshaded area of the boat deck on
R.R.S. Discovery.
3. Results and discussion
3.1. Analytical performance at sea
Forty-three depth pro®les were obtained during the
course of the cruise from a variety of different ocea-
nographic environments within the western Mediter-
ranean basin. The practical limit of detection (signal-
to-noise�3) while at sea was 10.6 nM H2O2 and the
reproducibility (relative standard deviation; RSD) of
two different surface samples was 2.5% at Station D10
at 11.18E, 37.48N (mean concentration�83.6 nM
H2O2; n�3) and 1.8% at Station MA7 at 4.98E,
43.18N (mean concentration�42.5 nM H2O2; n�4).
Analysis of a depth pro®le of 12 samples and standard
additions required 45 min and within-run repeatability
was <5% RSD (each sample or standard addition was
injected 4±6 times). No serious technical problems
were encountered during the course of the cruise.
3.2. Depth profile data
The weather during the cruise was generally very
settled and warm with minimal rainfall. There was
very little cloud cover but Mistral induced storms were
encountered off the south coast of France. The high
level of incident radiation produced detectable con-
centrations of H2O2 at all of the stations studied. All 43
depth pro®les obtained exhibited characteristic sur-
face maxima (ranging from 16.0±154 nM) followed
by a decrease in concentration with depth until the
practical limit of detection was reached (usually at 40±
60 m). Hydrogen peroxide was not observed in any of
the depth pro®les below 100 m (i.e. <10.6 nM). The
depth pro®le results are generally in agreement with
those reported previously for different water masses,
as shown in Table 1. Typical depth pro®les for meso±
oligotrophic waters at Stations MA1 and MD1 are
shown in Fig. 3 and the corresponding temperature
depth pro®les are shown in Fig. 4. The H2O2 surface
maxima were related to the higher photochemical
activity in surface waters and the decrease in concen-
tration with depth to three additive processes; dilution
of H2O2 enriched surface seawater with lower con-
centration water from below, consumption of H2O2 on
reaction with natural and anthropogenic chemical
species and the increased attenuation of incident
radiation decreasing the rate of H2O2 photo-genera-
Fig. 2. Optimised flow injection manifold with chemiluminescence
detection for the shipboard determination of hydrogen peroxide.
148 D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155
tion. These processes resulted in the establishment of
concentration gradients between the photo-active sur-
face waters and deeper waters controlled by the extent
of physical mixing, chemical reactivity and incident
radiation. The close relationship between H2O2 and
temperature depth pro®les at Stations MA1 and MD1
(Figs. 3 and 4), particularly the sharp decrease in
concentration at the thermocline, clearly demonstrates
that H2O2 is affected by the extent of wind induced
mixing.
3.3. Diurnal variations
Fig. 5 shows the results of a Lagrangian drift mode
diurnal study of H2O2 concentration at Station MA1
(6.08E, 41.08N) on 18th July 1993. Light measure-
ments for the day of study were obtained from a
deckbased PAR meter at the bow of the ship and these
data showed that there was very little cloud cover
during the course of the experiment, that sunrise was at
0400 and that maximum light intensity occurred at
1200. The diurnal variations of H2O2 concentration at
selected depth intervals (surface (1.1±1.7 m), 15 m
(14.0±15.3 m) and 40 m (40.0±44.4 m)) are shown
in Fig. 6. These diurnal variations assume that there
was no vertical transport of water between the selected
depths.
The surface water H2O2 concentration was rela-
tively constant (120±130 nM) from 0450 (only 50 min
after sunrise) until 1521. From this time onwards H2O2
at the surface was consumed at a greater rate than its
generation and the concentration decreased to a mini-
mum at 0027 on the next day (84 nM H2O2). The other
depth intervals showed similar generation and decom-
position cycling of H2O2 although the sub-surface
variations were less pronounced than at the surface.
The total H2O2 diurnal pro®le gave a concentration
maximum just after midday and rapidly decreased
thereafter. The sequence of pro®les from 1005 to
0027 demonstrates the potential use of H2O2 measure-
ments as a short-term tracer for vertical transport in the
Table 1
Hydrogen peroxide marine field data
Location [H2O2] (nM) Reference
North Atlantic <180 [1]
Gulf of Mexico <150 [2]
Gulf of Mexico <150 [10]
Atlantic 488N to 638S <175 [11]
Sargasso Sea <150 [12]
Peru Upwelling area <50 [18]
Western Mediterranean <150 [19]
Great Barrier Reef <110 [20]
Eastern Caribbean and
Orinoco plume
<420 [21]
Seto Inland Sea <400 [22]
North Atlantic, off coast
of SE USA
<125 [23]
Southern Ocean 498S to 628S <20 [24]
South and Equatorial Atlantic <240 [25]
Western Mediterranean and
RhoÃne plume
<160 This study
Fig. 3. Hydrogen peroxide depth profiles for Station MA1 (68E,
418N) at 12h04 local time on 20th July 1993 and Station MD1
(48E, 438N) at 16h40 local time on 31st July 1993.
Fig. 4. Temperature depth profiles for Station MA1 (68E, 418N) at
12h04 local time on 20th July 1993 and Station MD1 (48E, 438N)
at 16h40 local time on 31st July 1993.
D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155 149
water column. The increased H2O2 concentration at
ca. 30 m at MA1/9 was probably due to entrainment of
H2O2-rich surface water. The H2O2 anomaly was
slowly assimilated into the sub-surface waters and
traced by pro®les MA1/9 to MA1/14. The potential
of H2O2 measurements at sea to examine vertical
motions near the sea surface was ®rst proposed by
Johnson et al. [19] who observed large anomalies in
Fig. 5. Diurnal variations in hydrogen peroxide depth profiles at station MA1.
150 D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155
H2O2 concentrations at frontal regions of the western
Mediterranean (028W±038E, 36±378N) that were well
correlated with the salinity gradient which was known
to drive the frontal circulation.
The MA1 diurnal cast data was used to calculate net
H2O2 generation rates (when the concentration
between casts increased) and consumption rates (when
the concentration between casts decreased). The
ranges of generation and consumption rates were
0.0201±0.0315 nM H2O2 minÿ1 and (ÿ)0.0111±
(ÿ)0.0975 nM H2O2 minÿ1, respectively, as shown
in Table 2. These rates are in good agreement with
those reported previously, e.g. Moffett and Za®riou
[26] calculated generation rates in un®ltered coastal
waters of 0.045±0.057 nM minÿ1 when exposed to
¯uorescent light and 0.083±0.100 nM minÿ1 under
ambient sunlight and Johnson et al. [19] reported
consumption rates in the western Mediterranean of
(ÿ)0.050±(ÿ)0.120 nM minÿ1. The mean generation
and consumption rates at depth (40.0±44.4 m) were
less than at the surface (1.1±1.7 m) due to enhanced
biological and photochemical activity in the shallow
waters for both generation [12,27] and consumption
[14,15] processes. Diurnal variations in the surface
waters at Station MA1 in the western Mediterranean
(84±123 nM H2O2) were in excellent agreement with a
similar study in the Gulf of Mexico that reported 100±
130 nM H2O2 [10]. Both waters were relatively oli-
gotrophic but larger diurnal trends were reported for
more productive waters, e.g. the H2O2 concentration
in Jack's Lake, Ontario, peaked at 200±400 nM H2O2
in the late afternoon and decreased to <10 nM during
the night [8] while in more marine in¯uenced water
(Patuxent River Estuary, Maryland) H2O2 varied from
180 to 350 nM H2O2 [28]. The variation in H2O2
concentration ranges is likely to be due to matrix
differences between the fresh, estuarine and marine
waters which affect the light quality and penetration
characteristics (e.g. the extent to which light attenua-
tion is dependent on the concentration of suspended
solids, which tends to be higher in estuarine waters),
physical mixing, biological activity (e.g. with respect
to catalase activity) and the type and concentration of
H2O2-generating dissolved organic chromophores.
3.4. Geographical distribution of H2O2
As described above, the time of day greatly affected
the concentration of H2O2 in the sampled waters.
However, a ®rst order comparison of the geographical
variations of H2O2 was made by classi®cation of the
depth pro®le data into ®ve geographical areas. The
geographical distribution of H2O2 in the western
Mediterranean is shown in Table 3. Depth intervals
of 0±20 m, 21±40 m, 41±80 m and >80 m were chosen
to assess the depth pro®le characteristics from the
different areas. The population sizes for each geogra-
phical area were not consistent, e.g. the Straits of
Gibraltar area comprised only 24 data points, of which
15 were samples from below 80 m depth. However, a
number of ®ndings were evident from this limited data
set.
1. The highest H2O2 concentrations were determined
in the Straits of Sicily ([H2O2]max�143 nM) and
the inner RhoÃne ([H2O2]max�154 nM) areas. An
interesting result was that the highest H2O2
Fig. 6. Diurnal variations of hydrogen peroxide at different depth
intervals at Station MA1 on 18th July 1993.
Table 2
Net generation or consumption of H2O2 at Station MA1
Time period
(local time)
Generation (�) or consumption (ÿ) of H2O2
(nM minÿ1)
1.1±1.7 m 14.0±15.3 m 40.0±44.4 m
0450±1252 ÿ0.021 n/a n/a
0450±1005 n/a ÿ0.029 ÿ0.011
1005±1252 n/a ÿ0.070 �0.030
1252±1521 �0.020 �0.032 �0.031
1521±1950 ÿ0.043 ÿ0.028 ÿ0.024
1950±0027 ÿ0.098 ÿ0.064 ÿ0.023
D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155 151
concentration (154 nM) determined throughout
the entire period of the cruise (Cast MA8/1) was
collected at the shallowest recorded sampling
depth (0.2 m) implicating a possible micro-layer
enhancement of H2O2.
2. The oligotrophic classified stations had higher
mean H2O2 concentrations than the other four
geographical areas for the 0±80 m depth ranges.
The Straits of Sicily casts also had high H2O2
concentrations, particularly in the top 20 m of
the water column. The decrease in concentration
with depth is well demonstrated and was in agree-
ment with the typical depth profiles shown earlier
(Section 3.2). At depths >80 m very low H2O2
concentrations were typical at all five areas (mean
concentration �13 nM).
3. Although productivity data is unavailable, it was
assumed that the most productive waters were
those closest to the RhoÃne plume. However, the
less productive waters (oligotrophic and Straits of
Sicily) had the highest H2O2 concentrations. A
contributing factor was the fact that the oligo-
trophic and Straits of Sicily Stations were further
south than the RhoÃne Stations and therefore
received a greater light flux for the photochemical
generation of H2O2.
3.5. Photogeneration of H2O2
The photogeneration of H2O2, the most dominant
generation process of H2O2 in the marine environ-
ment, was investigated at sea and in the laboratory.
The effect of water origin on the photogeneration of
H2O2 was examined using shipboard incubations and
ambient sunlight (Fig. 7). The eutrophic/mesotrophic
RhoÃne plume surface sample (MA9) generated by far
Table 3
Geographical distribution of hydrogen peroxide in the western Mediterranean
Depth (m) Number of data points [H2O2]Mean (nM) [H2O2]Min (nM) [H2O2]Max(nM) S.D. (nM)
Straits of Gibraltar (D3 and D5)
0±20 2 33 20 46 13
20±40 2 21 17 25 4
40±80 5 18 13 23 3
>80 15 13 11 16 3
Oligotrophic (D8, D9, D16, MA1, MA2 and MA3)
0±20 34 91 51 136 21
20±40 17 61 33 88 15
40±80 40 28 11 65 12
>80 63 13 11 27 4
Straits of Sicily (D10, D11, D12, D12A, D14)
0±20 13 87 34 143 29
20±40 14 44 17 82 15
40±80 23 21 11 35 6
>80 44 13 11 46 6
RhoÃne, outer (MA4, MA5, MA6, MD1 and MF1)
0±20 49 72 19 123 23
20±40 32 28 11 76 14
40±80 30 13 11 23 4
>80 21 11 11 11 0
RhoÃne, inner (MA7, MA8, MA9, MF3 and ME2)
0±20 24 57 16 154 35
20±40 15 20 11 33 6
40±80 15 13 11 21 4
>80 7 11 11 11 0
152 D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155
the greatest concentration of H2O2 (401 nM after
360 min), probably due to River RhoÃne derived com-
ponents increasing the photochemical generation
potential of the sample. These photochemically active
components were assumed to be terrestrial in origin,
with humic and/or fulvic acids being the most likely
chromophores as these chemical species play an
important role in many natural aquatic photochemical
processes as light absorbing compounds [29]. The
deeper MA9 sample (39.4 m) generated signi®cantly
less H2O2 during the incubation (80 nM in 360 min).
This large difference over only 40 m of depth corre-
lated with differences in other water properties (e.g.
the salinity of the surface sample was 35.1 while at
39.4 m depth the salinity was 38.1) and was due to a
lack of vertical mixing between the plume in¯uenced
surface water and the deeper, denser Mediterranean
seawater at MA9. The surface D15 sample generated
slightly more H2O2 than the deep (2800 m) sample but
much less than the corresponding surface sample at
MA9. Lower concentrations of terrestrially derived
humic and fulvic compounds is the most likely reason
for the decreased photoreactivity of the surface D15
sample. This is corroborated by the more geographi-
cally remote location of D15 than MA9 (see Fig. 1).
3.6. Effect of molecular size on photogeneration
The effect of molecular size of the dissolved organic
matter in seawater on its potential to generate H2O2
was investigated by incubating ultra-®ltered samples
(see Section 2.2) as described in Section 2.4. The
dissolved organic carbon (DOC) content of each
fraction, as determined by two independent methods,
was 4.9% for the >8000 Da ®ltrate, 24.5% for the 300±
8000 Da fraction and 62.1% for the <300 Da ®ltrate
(total recovery 91.5%). The results for a 660 min
incubation are shown in Fig. 8. The data for the
300±8000 Da fraction have been corrected for the
420, 540, 600 and 660 min measurements because
the CL emission was off scale (>12 V) and assumes
that there was 100% recovery for the three fractions
compared with ®ltered seawater.
The 300±8000 Da fraction generated 62.3% of the
total H2O2 produced at a rate of
0.64 nM (mg C)ÿ1 hÿ1 while only accounting for
24.5% of the total DOC content. It can therefore be
deduced that the majority of the most ef®cient H2O2
generating chromophoric molecules were 300±
8000 Da in size and that the concentration of DOC
did not directly correlate with the amount of H2O2
generated. This 300±8000 Da fraction of Rhone plume
origin contained terrestrially derived fulvic and low-
to-medium molecular weight humic acids and a vari-
ety of substituted and unsubstituted aromatic and
medium-to-long chain aliphatic compounds. The
<300 Da fraction accounted for 62.1% of the total
DOC and 30.4% of the total H2O2 generated but gave
the lowest H2O2 generation rate of
0.12 nM (mg C)ÿ1 hÿ1 . This fraction comprised a
variety of low molecular weight inorganic and organic
Fig. 7. Effect of water origin on the photogeneration of H2O2.
MA9 samples were collected in the RhoÃne plume (58E, 438N) and
D15 samples in oligotrophic waters (68E, 408N).
Fig. 8. Shipboard photogeneration of H2O2 in filtered seawater and
in size fractionated seawater. Samples were collected from 1.64 m
depth at Station MA9 (58E, 438N) and incubated for 540 min from
09h30 on 29th July 1993.
D. Price et al. / Analytica Chimica Acta 377 (1998) 145±155 153
species, e.g. nitrates, carbohydrates, hydrogen carbo-
nates and short chain aliphatic compounds. The
>8000 Da fraction accounted for 4.9% of the total
DOC and 7.2% of the total H2O2 generated but the
generation rate of 0.38 nM (mg C)ÿ1 hÿ1 was higher
than for the <300 Da fraction. This suggests that the
large molecular weight humic acids in this fraction
were moderately photoreactive.
The link between H2O2 generation ef®ciency and
humic/fulvic acid concentration was further investi-
gated in the laboratory by irradiating aqueous solu-
tions of river Dodder humic acid (DHA), fulvic acid
(DFA) and hydrophilic macromolecular acid (HMA)
and a commercial ionic humic acid salt (AHA) with a
solar simulator (1.8 kW Xe-arc lamp) for 180 min at
full power (194.0 W mÿ2). The results (Fig. 9)
showed that each organic fraction generated consider-
able quantities of H2O2 during the course of the
incubation with the fulvic acid fraction being 50%
less ef®cient than the other three. Additional incuba-
tions resulted in a linear relationship between humic/
fulvic acid concentration and H2O2 generation over
the range 0.05±2.0 mg lÿ1 (r�0.9411±0.9915; n�5).
There was also a small but measurable increase in
H2O2 from incubation of a Milli-Q water blank
(0.92 nM minÿ1). These results were in broad agree-
ment with the shipboard incubation data reported
above and suggest that ®eld based H2O2 measure-
ments may be useful as a surrogate for the photode-
gradation rate of DOC.
4. Conclusions
Shipboard results con®rm the feasibility of using a
¯ow injection-chemiluminescence method for H2O2
in seawater. The instrumentation is well suited to the
harsh conditions of deployment at sea and the analy-
tical ®gures of merit show excellent repeatability
(<5%) and reproducibility (1.8±2.5%). Hydrogen per-
oxide depth pro®les exhibited characteristic surface
maxima followed by a rapid decrease with depth to
below the practical limit of detection beyond the
thermocline. The pro®les were affected by the amount
of incident radiation, physical mixing processes and
seawater composition. Hydrogen peroxide depth pro-
®les varied diurnally, with surface water concentra-
tions showing the most signi®cant variation. At
Station MA1 in the central western Mediterranean
the concentration of H2O2 ranged from 84±123 nM.
Hydrogen peroxide generation and consumption rates
were in good agreement with previous studies. Ter-
restrially in¯uenced waters were more ef®cient at
photogenerating H2O2 than less productive waters
offshore in deck based incubations and laboratory
studies indicated that humic compounds are important
precursors in the formation of H2O2.
Acknowledgements
The authors wish to acknowledge the Natural Envir-
onment Research Council and Plymouth Marine
Laboratory for the award of a NERC CASE student-
ship to D.P., Lulwa Ali (UoP), Tim Fileman (PML)
and Jun Zhou (PML), and the crew, of®cers and staff
of Research Vessel Services while onboard the R.R.S.
Discovery during cruise D203. We would also like to
thank Guenther Fengler and Alessandro Spitzy (Uni-
versity of Hamburg) for the fractionated seawater
samples.
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